measure voltage at the alt. with your Fluke.  The regulator is integral
to the alternator.  A lot of times there's enough voltage drop that you
can read 12V at the dash gauge and still be getting the correct 13.7 at
the back of the alternator.  Now what is a problem is if you aren't
getting 13.7 at the battery - clean your cables if you have correct
voltage at the alt. but not the batt.  replace if necessary.  also check
all ground points for corrosion and clean as necessary.  Finally I think
13.7 is too low, I'd like to see at least 14V but that is the Bosch
spec.  You can "hotrod" your alt. a little by getting a Transpo brand
regulator with adjustable voltage setpoint, I thin kthey come set around
14.1V which is a nice compromise between high voltage to make everything
work well and not overcharging the battery.

good luck,

nate

Yes I believe the 80 Rabbit alternators are basically insulated from
directly contacting the metal and getting a good ground.  I still have some
of those bushing kits too!  lol
There is usually some thick wiring to ground the alternators to the block or
a metal bracket.
And especially if the Rabbit has stock A/C.

I would add a ground to that alternator and check the voltage.
If it still charges at a low level check the wiring plug to the alternator.
Also there are some auto part stores that will check your alternator for
free!  ;-)

BTW Is this a Motorola or a Bosch alternator?  Both should have an internal
regulator so it should come with the alternator.